The present invention relates to a halogenated polysilane as a pure compound or a mixture of compounds each having at least one direct Si—Si bond, whose substituents consist exclusively of halogen or of halogen and hydrogen and in the composition of which the atomic ratio of substituent to silicon is at least 1:1.
Chlorinated polysilanes of this kind are known in the prior art [references: DE 10 2005 024 041 A1; DE 10 2006 034 061 A1; WO 2008/031427 A2; WO 81/03168; US 2005/0142046 A1; M. Schmeisser, P. Voss “Über das Siliciumchlorid [SiCl2]x”, Z. anorg. allg. Chem. (1964) 334, 50-56 (Schmeisser, 1964); US 2007/0078252 A1; DE 31 26 240 C2; UK 703,349]. On the one hand they can be produced in a purely thermal reaction (Schmeisser, 1964) by heating chlorosilanes in vapor form with or without a reducing agent at relatively high temperatures (above 700° C.). The chlorinated polysilanes obtained have a faint coloration from dirty-yellow to yellowish-light-brown (Schmeisser, 1964; “slightly greenish-yellow, glass-like, high-polymer”). Spectroscopic investigations have shown that these polysilanes produced purely thermally have a high proportion of short-chain, branched and cyclic molecules. Furthermore, the mixture obtained is heavily contaminated with AlCl3 owing to the production conditions (very high temperatures).
GB 702,349 discloses that when silicon alloys are reacted with chlorine gas at 190-250° C. a mixture of chlorinated polysilanes is condensed from the gas stream. The average molecular weight of these mixtures is relatively low, because on distillation only 2% of the silanes have n greater than 6.
DE 31 26 240 C2 describes the wet-chemical production of chlorinated polysilanes from Si2Cl6 by reaction with a catalyst. The mixtures obtained still contain the catalyst and are therefore washed with organic solvents, so that traces of this solvent and of the catalyst remain. Moreover, PCS obtained in this way are strongly branched. Other wet-chemical methods are presented in US 2007/0078252 A1;                1. Reduction of halogenated aryloligosilanes with sodium followed by cleavage with HCl/AlCl3 aromatics.        2. Transition-metal-catalyzed dehydrogenating polymerization of arylated H-silanes and subsequent dearylation with HCl/AlCl3.        3. Anionically catalyzed ring-opening polymerization (ROP) of (SiCl2)5 with TBAF (Bu4NF).        4. ROP of (SiAr2)5 with TBAF or Ph3SiK and subsequent dearylation with HCl/AlCl3.        
With all these methods, once again PCS contaminated with solvent/catalyst are obtained.
In H. Stüger, P. Lassacher, E. Hengge, Zeitschrift für allgemeine and anorganische Chemie 621 (1995) 1517-1522, Si5Cl9H is converted to the corresponding bis-cyclopentasilane Si10Cl18 by boiling with Hg(tBu2) in heptane. Alternatively, cyclization of Si5Ph9Br with naphthyllithium or K and/or Na/K can be performed in various solvents with subsequent halogenation with HCl/AlCl3.
Production of halogenated polysilanes of this kind by a plasma-chemical process is also known. For example, DE 10 2005 024 041 A1 relates to a method of production of silicon from halosilanes, in which in a first step the halosilane is converted to a halogenated polysilane with production of a plasma discharge, and this is then decomposed to silicon in a second step, with heating. In this known method, in connection with production of the plasma, high energy densities are used (above 10 Wcm−3), and the end product is a not very compact, waxy-white to yellowish-brown or brown solid. Spectroscopic investigations showed that the end product obtained has a relatively high degree of crosslinking. The high energy densities used lead to products of high molecular weights, resulting in insolubility and low fusibility.
Furthermore, WO 81/03168 describes a high-pressure plasma process for synthesis of HSiCl3, in which PCS are formed as minor by-products. As these PCS are produced at extremely high gas temperatures, they are relatively short-chained and strongly branched. Moreover, this PCS has a high H content, owing to the hydrogenating conditions (HSiCl3 synthesis!). US 2005/0142046 A1 describes PCS production by silent electric discharge in SiCl4 at normal pressure. Only short-chain polysilanes are formed, as is demonstrated by the author with the selective reaction of SiH4 to Si2H6 and Si3H8 by connecting several reactors in series. The situation is similar according to DE 10 2006 034 061 A1, in which a similar reaction is described, in which gaseous and liquid PCS are obtained, with Si2Cl6 as the main constituent (p. 3, [00161]). Admittedly the authors describe how the molecular weights of the PCS can be increased by using several reactors connected in series, but it is only possible to obtain material that can be converted undecomposed to the gas phase. This situation is also expressed by the authors in the claims, in which they envisage distillations for all PCS mixtures obtained.
In addition to chlorinated polysilanes, other halogenated polysilanes SixXy (X═F, Br, I) are also known in the prior art.
E. Hengge, G. Olbrich, Monatshefte für Chemie 101 (1970) 1068-1073 describes the production of a polymer with a sheet structure (SiF)x. The sheet-structure polymers (SiCl)x or (SiBr)x are obtained from CaSi2 by reaction with ICl or IBr. A halogen exchange is then performed with SbF3. However, partial degradation of the Si layer structure occurs. The resultant product contains the amount of CaCl2 based on stoichiometry from CaSi2, and this cannot be washed out.
The production of polyfluorosilane (SiF2)x is described for example in M. Schmeisser, Angewandte Chemie 66 (1954) 713-714. SiBr2F2 reacts at room temperature in ether with magnesium to a yellow, high-polymer (SiF2)x. Compounds such as Si10Cl22, (SiBr)x and Si10Br16 can be rehalogenated with ZnF2 to the corresponding fluorides. The standard method of production of (SiF2)x is presented for example in P. L. Timms, R. A. Kent, T. C. Ehlert, J. L. Margrave, Journal of the American Chemical Society 87 (1965) 2824-2828. In this, (SiF2)x is produced by leading SiF4 over silicon at 1150° C. and 0.1-0.2 torr and freezing out the resultant SiF2 at −196° C. with polymerization during subsequent thawing. The colorless to pale yellow plastic polymer melts on heating to 200-350° C. in vacuum and releases perfluorinated silanes from SiF4 to at least Si14F30. A silicon-rich polymer (SiF)x remains, which at 400±10° C. decomposes vigorously to SiF4 and Si. The lower perfluoropolysilanes are colorless liquids or crystalline solids, which can be isolated by fractional condensation in purities >95%.
Traces of secondary or tertiary amines catalyze the polymerization of perfluoro-oligosilanes.
FI 82232 B discloses a reaction at even higher temperature. SiF4 reacts with Si in an Ar-plasma flame to SiF2 (0.8:1 mol, 70% SiF2 content).
Short-chain perbrominated polysilanes are formed, according to A. Besson, L. Fournier, Comptes rendus 151 (1911) 1055-1057. An electric discharge in gaseous HSiBr3 produces SiBr4, Si2Br6, Si3Br3 and Si4Br10.
K. Hassler, E. Hengge, D. Kovar, Journal of molecular structure 66 (1980) 25-30 prepare cyclo-Si4Br8 by reaction of (SiPh2)4 with HBr under AlBr3 catalysis. In H. Stüger, P. Lassacher, E. Hengge, Zeitschrift für allgemeine and anorganische Chemie 621 (1995) 1517-1522, Si5Br9H is converted to the corresponding bis-cyclopentasilane Si10Br18 by boiling with Hg(tBu2) in heptane. Alternatively, cyclization of Si5Ph9Br with naphthyllithium or K or Na/K can be performed in various solvents with subsequent halogenation with HBr/AlBr3.
High-molecular silicon sub-bromides can be prepared, according to M. Schmeisser, Angewandte Chemie 66 (1954) 713-714, on the one hand by reaction of SiBr4 with magnesium in ether in the form of the yellow, solid (SiBr)x, and on the other hand by the action of SiBr4 on elemental Si at 1150° C., which in addition to (SiBr)x also produces Si2Br6 and other oligosilanes such as Si10Br16.
DE 955414 B also discloses a reaction at high temperature. If SiBr4 or Br2 in vapor form are led in vacuum at 1000-1200° C. through silicon grit, there is mainly formation of (SiBr2)x, along with a little Si2Br6.
The ring-opening polymerization of cyclo-Si5Br10 and cyclo-Si5I10 by the action of Bu4NF in THF or DME is claimed in US 2007/0078252 A1.
For example E. Hengge, D. Kovar, Angewandte Chemie 93 (1981) 698-701 or K. Hassler, U. Katzenbeisser, Journal of organometallic chemistry 480 (1994) 173-175 report on the production of short-chain periodated polysilanes. By reacting the phenylcyclosilanes (SiPh2)n (n=4-6) or Si3Ph8 with HI under AlI3 catalysis, the periodated cyclosilanes (SiI2)n (n=4-6) or Si3I8 are formed. M. Schmeisser, K. Friederich, Angewandte Chemie (1964) 782 describe various routes for the production of periodated polysilanes. (SiI2)x is produced at approx. 1% yield when SiI4 vapor is led over elemental silicon at 800-900° C. at high vacuum. The pyrolysis of SiI4 in the same conditions gives the same product, which is very sensitive to hydrolysis and is soluble in benzene. Under the action of a glow discharge on SiI4 vapors at high vacuum, a solid, amorphous, reddish-yellow silicon sub-iodide of the composition (SiI2.2)x, insoluble in all the usual solvents, is obtained at a yield of 60 to 70% (relative to SiI4). Pyrolysis of this substance at 220 to 230° C. at high vacuum leads to a dark red (SiI2)x, with SiI4 and Si2I6 being formed simultaneously. The chemical properties of the resultant compounds (SiI2)x coincide—except for solubility in benzene. Pyrolysis of (SiI2)x at 350° C. at high vacuum produces SiI4, Si2I6 and an orange-red, friable solid with the composition (SiI)x. (SiI2)x reacts with chlorine or bromine between −30° C. and +25° C. to benzene-soluble, mixed silicon subhalides such as (SiClI)x and (SiBrI)x. At higher temperatures the Si—Si chains are cleaved by chlorine or bromine with simultaneous total substitution of the iodine. Compounds of the type SinX2n+2 (n=2-6 for X═Cl, n=2-5 for X═Br) are obtained. (SiI2)x reacts with iodine at 90 to 120° C. in a bomb tube completely to SiI4 and Si2I6.